1. Field of the Invention
The present invention relates to a detecting device of an encoder, especially to an optomechanical encoder used in a computer mouse or in a related pointing device.
2. Background of the Invention
The conventional mouse uses a detecting circuit with a conventional optomechanical encoder, as shown in FIG. 1. The detecting circuit is formed by a photodetector 11 and a detection circuit 12. The photodetector 11 is of the type of a phototransistor with two active areas P1 and P2 (as shown in FIG. 2) for receiving the optical signals from the working light beam through a code wheel 3, transferring the optical signals into electric signals, and then outputting the electric signals to a detection circuit 12. The detection circuit 12 is installed with two comparators OP1 and OP2. The input positive end (+) of the first comparator OP1 is connected with the output of the first active area P1 and further connected with a resistor R1 and the negative end (-) thereof is connected with resistors R2 and R3. The other end of the resistor R2 is connected with the other end of the resistor R1. The other end of the resistor R3 is connected with a power source VDD.
The input positive end of the second comparator OP2 is connected with the output of the second active area P2 and further connected with a resistor R4 and the negative end (-) thereof is connected with resistors R5 and R6. The other end of the resistor R5 is connected with another end of the resistor R4. The other end of the resistor R6 is connected with a power source VDD.
By the above connection, each of the input negative ends (-) of the comparators OP1 and OP2 may connect with a fixed reference voltage Vref1 and Vref2, wherein EQU Vref1=VDD.times.R2/R2+R3) EQU Vref2=VDD.times.R5/(R5+R6)
The active area P1 and P2 (as shown in FIG. 2) for receiving the optical signals from the working light beam through a code wheel 3 are easily affected by the sensitivity of the photodetector 11 and the light intensity of the light emitting source LED, and thus the output encoding signal is also affected, as shown in FIG 3. After comparing the level of the encoding signals with the fixed reference voltages Vref1 and Vref2 by the comparators OP1 and OP2, respectively, digital waveforms with different phase periods and pulse width are obtained, as shown in FIG. 4.
The aforementioned circuit has the following drawbacks in applications:
1) Since a fixed reference voltage (Vref) is used as a bigger voltage in the detecting circuit for digital wave shaping, the magnitude of the encoding signal could be too strong or too weak, depending on the sensivities of the photodetectors and the light intensity of the light source LED. Therefore, it is necessary to classify and match different LEDs and photodetector
Since the reference voltage in the detecting circuit is performed by a fixed voltage, it is difficult to control the the duty cycle of the output signals (A, B) and the phase differences are difficult to be controlled. Therefore, the vector data can not be provided correctly. If this occurs in a mouse, the mouse will lose counts and become motionless.
2) The manufacture of the photodetectors and LEDs must classify and match the photodetectors and LEDs correctly in order to satisfy the requirement of the manufacturer of mice. Thus troubles in inventory and assembling are induced.
3) For the manufacture of mice, the produced mice are probably not good and the quality thereof is difficult to be controlled because of the assembly tolerance and decay of the LEDs in a mouse and the variation of the IC comparing voltage.